1. Field of the Invention
This invention relates to a sensor and associated circuitry for sensing and reacting to the presence of a fluid, a permeable medium, a fluid carried by a solid medium, or other substance having heat capacity, and to a method for using the sensor and associated circuitry. It also relates to an apparatus and method for performing a switching step when the amount of the fluid or other substance, or the concentration or flow rate of the fluid, reaches a certain predetermined level.
Typical applications of the invention are as follows:
(a) Detecting the presence of rainfall or dispensed water upon an area of land served by an irrigation system;
(b) Sensing and reacting to leakage, spillage or overflow of oil within a wall surrounding an oil-storage tank;
(c) Detecting and reacting to leakage of liquid from a conduit into insulation or another substance; and
(d) Sensing and reacting to a threshold rate of flow of liquid through a conduit.
When the quantity of rainfall or dispensed water, or the spillage of oil, or the leakage of liquid, or the rate of liquid flow reaches a predetermined level, appropriate circuitry produces an electrical signal to operate an alarm or suitable control equipment as required by the circumstances.
Preferably, the sensor comprises at least one positive-temperature-coefficient (xe2x80x9cPTCxe2x80x9d) device coupled to single or plural signal-processing circuits for actuating any desired output apparatus for indication or control.
2. Description of the Prior Art
Attention is invited to my copending application Ser. No. 09/221,733, filed on Dec. 28, 1998 and entitled xe2x80x9cMethod and Apparatus for Sensing and Measuring Plural Physical Properties, Including Temperature.xe2x80x9d That application discloses an arrangement including at least one xe2x80x9ctabletxe2x80x9d of positive-temperature-coefficient (xe2x80x9cPTCxe2x80x9d) material with a plurality of zones, which have some electrical dependence upon each other. The arrangement permits the measurement of at least one physical property, one of which may be temperature.
If a property, or the presence of a substance, is to be sensed, but not necessarily measured, the circuitry may be simpler and less expensive than that which is illustrated in the aforementioned copending application. Such a less-expensive configuration is disclosed and claimed in the present specification.
Typically, PTC material comprises a mixture of barium and/or strontium titanates suitably xe2x80x9cdopedxe2x80x9d with certain trivalent or pentavalent elements which serve to adjust the temperature at which the material reaches its xe2x80x9cCurie point.xe2x80x9d At about the Curie point, a plot of electrical resistance of the material as a function of its temperature becomes very steep as temperature increases further. At still higher temperatures, the plot levels off at a xe2x80x9ckneexe2x80x9d as shown in FIG. 1 of the drawings of this specification. The characteristics of PTC materials are well described in the 1996 Supplemental Catalogue of Keystone Thermometrics, of St. Mary""s, Pa., a copy of which is made a part of the file of this specification. The most pertinent disclosures appear on pages 31 through 38 of the Catalogue.
An xe2x80x9cAtmospheric Sensorxe2x80x9d employing PTC material is shown and described in U.S. Pat. No. 4,890,494xe2x80x94Osbond et al, which is also entered in the file of this specification. That patent discloses a probe of PTC material for measuring the liquid content of a gas. But it makes no mention of a switching circuit for turning on or off an indication or function. Furthermore, Osbond et al do not reveal a sensor which may comprise a single tablet of PTC material divided into zones which are nevertheless electrically and thermally dependent upon each other.
In view of the distinctions of the present invention over the prior art, I have provided a sensing and switching circuit which is new in its concept and surprising in its capabilities, while employing a modest amount of hardware.
The sensing circuit in accordance with the present invention is built around a tablet of PTC material to which are bonded, preferably on a respective first side and second side thereof, first and second layers of ohmic resistive (or conductive) material. While the first such layer is usually continuous in configuration, the second layer is separated into a first zone and a second zone which are not in direct electrically-conductive relationship with each other. The first layer is connectable, through switching or other means, to a first source of electric potential xe2x80x9c+V.xe2x80x9d The second zone of the second layer is connected through a resistor to ground, or connected through a resistor to a second source of different electric potential.
The tablet of PTC material and the first zone of the second layer are in physical contact with a body which, from their standpoint, is a heat sink. The body may, for example, comprise a diaphragm on the opposite side of which may be present (or not be present) drops of water or other fluid. The first zone of the second layer may be electrically connected through first and second series-resistor means to ground or a source of different electric potential.
The junction or node between the first and second series-resistor means may be coupled to the input of a switching device such as an NPN transistor. The output terminals of the switching device may be connected through a gating device to an alarm, a signal light, a control valve, a motor switch, a meter, or other output device.
Means may be provided for continuously or periodically applying the voltage xe2x80x9c+Vxe2x80x9d to the first layer of ohmic material. When the voltage xe2x80x9c+Vxe2x80x9d is first applied to the first layer, the temperature and resistance of the tablet of PTC material are low, and the current through it immediately becomes high. But, as the current warms the tablet, the portion of it which is in contact with the second zone of the second layer reaches the Curie point and sharply increases in resistance. Accordingly, the current flowing through the second zone of the second layer and the portion of the tablet of PTC material proximate thereto will sharply decrease.
The portion of the tablet of PTC material which is in contact with the heat sink and with the first zone of the second layer of ohmic resistive material will warm up more slowly than the portion of the tablet proximate the second zone of the second layer. And if the heat sink includes a diaphragm carrying drops of water, oil, or other liquid of high heat capacity, the portion of the tablet proximate the first zone of the second layer will warm up still more slowly. The warm-up time as seen through the first zone of the second layer, when compared with the warm-up time as seen through the second zone of the second layer, will be substantially greater. Moreover, the time disparity will increase with the concentration of heat-absorbing water or oil or other material (the xe2x80x9cheat sinkxe2x80x9d) on the opposite side of the diaphragm or other structure that is in physical contact with the portion of the tablet of PTC material proximate the first zone of the second layer of ohmic resistive material.
In accordance with the present invention, the attainment and surpassing of the Curie point by the portion of the tablet of PTC material which is proximate the heat sink and the first zone of the second layer of ohmic material is an event which can initiate a switching action. When that portion of the tablet reaches the Curie point, the current through it and its associated first and second series-resistor means drops sharply, and the current available from the node between the series-resistor means becomes insufficient to maintain the transistor in its conductive state. The transistor therefore turns off. This switching action in response to the state of the heat sink is one of the most useful features of this invention.
Also in accordance with the present invention, controlled heat flow from the portion of the tablet proximate the second zone to the portion of the tablet proximate the first zone is useful in establishing the thermal and electrical conditions required for initiation of the switching action. Such controlled heat flow may be referred to as a xe2x80x9cthermal boost.xe2x80x9d